Circuit arrangement for limiting the current at make of a capacitative load

ABSTRACT

A circuit arrangement for limiting the current at make which includes hot-carrier thermistors that see to a reduction of the current flow at make. Only when a further hot-carrier thermistor has been adequately heated by the flow of current are the hot-carrier thermistors bridged, and the capacitative load receives the full operating voltage. After turn-off, an ambient temperature-compensated voltage divider at the base of a transistor prevents the immediate re-activation until the further hot-carrier thermistor and, thus, all hot-carrier thermistors, have adequately cooled.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a circuit arrangement for limitingthe current at make of a capacitative load that is operated with anoperating voltage.

2. Description of the Prior Art

When switching on a capacitative load such as, for example, servo outputstages for driving X-Y motors, the capacitative load charges suddenly.Given support capacitors (up to 18000 μF) of the output stages, currentpeaks up to 200 A can occur that fuse the contacts of contactors thatintervene in the circuit for safety reasons. These contactors are, thus,no longer functional.

An object of the present invention, therefore, is to specify a circuitarrangement for limiting the current at make of a capacitative load thatreduces the currents flowing during the turn-on event.

SUMMARY OF THE INVENTION

Such object is achieved in accordance with the teachings of the presentinvention wherein two switches that switch at different points in timeare advantageously employed. The first switch switches the current via ahot-carrier thermistor that is connected between operating voltage andcapacitative load. As a result, the current at make is limited andincreases in its time curve by intrinsic heating of the hot-carrierthermistor. The second switch switches after a delay time relative tothe first switch and, thus, effects that the hot-carrier thermistor isbridged at the operating voltage and is available to the capacitativeload.

In an embodiment of the present invention, an especially simple circuitarrangement is employed by the further hot-carrier thermistor thateffects the delay time.

In another embodiment of the present invention, in order to avoid acurrent surge during turn-on after the hot-carrier thermistors are stillheated, it is provided that the first and the second switch are formedas a relay, switching in common, whose supply voltage is only switchedwhere the further hot-carrier thermistor and, thus, the otherhot-carrier thermistor have already cooled.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the Detailed Description of thePreferred Embodiments and the Drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit arrangement for limiting the current at make of acapacitative load.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 1, a capacitative load 1 is connected to an operatingvoltage 2 via a rectifier 3 as well as a circuit of contactors 4, 10,20. The inputs of the first contactor 4 are thereby connected to theoperating voltage 2. Given a three-phase supply, it is a matter of threeinputs for which the respectively same current paths apply. One currentpath is described; the other two current paths are correspondinglycircuited. The first contactor 4 is equipped with switches 5 that areswitched by an activator means 6 when an activation voltage is adjacentat the activator input 7 and at the activator output 8. The inputs ofthe first contactor 4 are connected to the outputs of the firstcontactor 4 via the switches 5. The outputs of contactor 4 are connectedto some of the inputs of a second contactor 10 that is equipped withswitches 11 and a further switch 15. The switches 11 and the furtherswitch 15 are switched by an activator means 12 of the second contactor10 when an activation voltage is adjacent at both the activator input 13and the activator output 14 of the second contactor 10. The outputs ofthe second contactor 10 are connected to the inputs of the rectifier 3that rectifies the adjacent three-phase current voltage and applies itto the capacitative load 1; for example, the servo output stage of X-Ymotors.

A series circuit of a third contactor 20 and hot-carrier thermistors 40is connected between the output of the first contactor 4 and therectifier 3 and parallel to the second contactor 10. The hot-carrierthermistor 40 exhibits a temperature-dependent impedance behaviorwherein the impedance decreases with increasing temperature (NTC:negative temperature coefficient). To that end, the outputs of the firstcontactor 4 are connected to the inputs of the third contactor 20, andthe inputs of the third contactor 20 are connected via switches 21 tothe outputs of the third contactor 20. The switches 21 are switched byan activator means 22 in the third contactor 20 when an activationvoltage is adjacent at the activator input 23 and at the activatoroutput 24.

The hot-carrier thermistors 40 are arranged in a limiting means 33 forcurrent at make and are connected via terminals 39 to the output of thethird contactor 20 and to the outputs of the second contactor 10.

Two control voltages 30, 31 that, for example, both amount to 24 V DCvoltage compared to the ground 32, serve for the control of thecontactors 4, 10, 20. The terminal of the first control voltage 30 isconnected to the activator inputs 7, 13, 23 of the first 4, second 10and third 20 contactors, respectively, as well as to a first terminal 34of the limiting means 33 for current at make. The activator outputs 8,24 of the first 4 and third 20 contactors, respectively, are connectedto a second terminal 35 of the limiting means 33 for the current atmake. The second terminal 35 of the limiting means 33 for the current atmake is applied, via a first switch 42 of a relay 41, to a thirdterminal 36 of the limiting means 33 for the current at make that isconnected to the ground line 32.

The activator output 14 of the second contactor 10 lies at a fourthterminal 37 of the limiting means 33 for the current at make. Thisfourth terminal 37 is connected via a second switch 43 of the relay 41and via a further hot-carrier thermistor 44 connected in seriestherewith to the third terminal 36 of the limiting means 33 for thecurrent at make.

The activator output 14 of the second contactor 10, over and above this,is connected to a further output 17 of the second contactor 10, whereinthe further output 17 is connected via the further switch 15 to afurther input 16 of the second contactor 10, and then the latter isconnected to the ground 32.

The terminal of the second control voltage 31 is connected to a fifthterminal 38 of the limiting means for the current at make, this beingconnected via a resistor 47 and an additional hot-carrier thermistor 48to the base of an npn-transistor 46. The emitter of the npn-transistor45 is applied via the third terminal 36 to the ground 32, whereas thecollector of the transistor 46 is adjacent the first terminal 34 of thelimiting means 33 for the current at make via a fuse 45. The secondterminal 35, the fourth terminal 37 and the collector of the transistor46 are respectively connected to the first terminal 34 via clinchingdiodes 49.

The function of the circuit arrangement is as follows: the first controlvoltage 30 serves as supply voltage of the relay 41 and the secondcontrol voltage 31 serves as control voltage of the relay 41. Wheneither one of the voltages 30, 31 is missing, the relay 41 is notexcited and the capacitative load 1 is not connected to the operatingvoltage 2. When the relay 41 switches, the first contactor 4 and thethird contactor 20 also switch simultaneously. Limited by thehot-carrier thermistors 40, current can flow via the rectifier 3 intothe load circuit 1. This limited current at make increases in its timecurve due to the intrinsic heating of the hot-carrier thermistors 40 Thecharging of the capacitative load 1 thus occurs in an avalanche-likemanner. The charging current, in turn, decreases with increasingcharging. Since operating current already flows with the cut-in, acertain voltage drops off at the hot-carrier thermistors 40. The voltagedrop-off is all the lower the lower the impedance of the hot-carrierthermistors 40. When the second contactor 10 bridges the hot-carrierthermistors 40, the current peak caused by the residual charging of thecapacitative load 1 is kept within harmless limits.

Simultaneously with the activator circuits of the first contactor 4 andthe third contactor 20, the relay 41 also closes the activator circuitof the second contactor 10. The current flow through this circuit heatsthe further hot-carrier thermistor 44. The further hot-carrierthermistor 44 becomes lower in impedance, the voltage drop-off at thefurther hot-carrier thermistor 44 sinks and the activator voltageincreases. When the switching threshold of the second contactor 10 isexceeded, the second contactor 10 bridges the hot-carrier thermistor 40.The electrical energy is thus available to the capacitative load 1without limitation.

The self-holding circuit via the further switch 15 holds the secondcontactor 10 in the activated condition. All hot-carrier thermistors 40,44 now cool uniformly, so that a re-activation with delay is possibleafter the turn-off of the first 4, second 10 and third 20 contactors.When, however, the cooling time is too short, so that the current surgeat make could become too high given reactivation, the ambienttemperature-compensated base voltage divider at the npn-transistor 46prevents the switching of the relay 41 via the additional hot-carrierthermistor 48 until the threshold of the base voltage is exceeded as aresult of the cooling of the further hot-carrier thermistor 44. Onlythen does the relay 41 switch, the first contactor 4 and the thirdcontactor 20 switch and, after a delay, the second contactor 10.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

We claim as our invention:
 1. A circuit arrangement for limiting thecurrent at make of a capacitative load that is operated with anoperating voltage, comprising:a first contactor having operating inputsto which the operating voltage is applied; a second contactor havingoperating inputs which are at least partially connected to the operatingoutputs of the first contactor, and having operating outputs connectedto the capacitative load; a third contactor having operating inputsconnected to the operating outputs of the first contactor; a pluralityof hot-carrier thermistors respectively circuited between the operatingoutputs of the third contactor and the capacitative load; a first switchthat closes the first and the third contactors; and a second switch thatcloses the second contactor after a delay time relative to the closingof the first and the third contactors.
 2. A circuit arrangement asclaimed in claim 1, further comprising:a further hot-carrier thermistorarranged in a supply circuit of the second contactor, wherein the delaytime is generated by the heating at the further hot-carrier thermistor.3. A circuit arrangement as claimed in claim 2, further comprising:arelay formed from the first switch and the second switch switching incommon; and a transistor having an ambient temperature-compensatedvoltage divider, wherein the relay draws supply voltage from the voltagedivider so that the relay is only switched when the further hot-carrierthermistor has cooled.